1. Field of the Invention
The invention relates to a solid electrolyte material used for an all-solid battery, an electrode element that includes a solid electrolyte material, an all-solid battery that includes a solid electrolyte material, and a manufacturing method for a solid electrolyte material.
2. Description of the Related Art
With a rapid proliferation of information-related equipment and communication equipment, such as personal computers, camcorders and cellular phones, in recent years, it becomes important to develop an excellent battery (for example, lithium battery) as a power source of the information-related equipment or the communication equipment. In addition, in fields other than the information-related equipment and the communication-related equipment, for example, in automobile industry, development of lithium batteries, and the like, used for electric vehicles or hybrid vehicles has been proceeding.
Here, existing commercially available lithium batteries employ an organic electrolytic solution that uses a flammable organic solvent. Therefore, it is necessary to install a safety device that suppresses an increase in temperature in the event of a short circuit or improve a structure or a material for short-circuit prevention. In contrast to this, all-solid batteries that replace a liquid electrolyte with a solid electrolyte do not use a flammable organic solvent in the batteries. For this reason, it is considered that the all-solid batteries contribute to simplification of a safety device and are excellent in manufacturing cost or productivity.
Such all-solid batteries include a positive electrode layer, a negative electrode layer and a solid electrolyte layer that is arranged between the positive electrode layer and the negative electrode layer. Thus, when the positive electrode layer is formed by powder molding using only a positive electrode active material, because the electrolyte is solid, it is difficult for the electrolyte to permeate into the positive electrode layer. This reduces the area of the interface between the positive electrode active material and the electrolyte, so battery performance deteriorates. Therefore, a positive electrode mixture formed of a mixture of the powder of a positive electrode active material and the powder of a solid electrolyte material is used to form a positive electrode layer to thereby increase the area of the interface. However, when the positive electrode mixture is used to form a positive electrode layer by powder molding, interface resistance against movement of lithium ions across the interface between the positive electrode active material and the solid electrolyte material tends to increase. It is believed that this is because the positive electrode active material reacts with the solid electrolyte material to thereby form a high-resistance portion on the surface of the positive electrode active material (see Narumi Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007), pages 1486 to 1490).
A sulfide-based solid electrolyte material is known as a solid electrolyte material. The sulfide-based solid electrolyte material has a high lithium ion conductivity, so it is useful to obtain a high-power battery. For example, Japanese Patent Application Publication No. 2005-228570 (JP-A-2005-228570) describes Li2S—P2S5-based crystallized glass as a sulfide-based solid electrolyte material. However, for the above described reason, even when the lithium ion conductivity of the sulfide-based solid electrolyte material is enhanced, it is still difficult to sufficiently obtain performance when the sulfide-based solid electrolyte material is used for the positive electrode mixture.
Then, in the existing art, there is an attempt to improve the performance of an all-solid battery by focusing on the interface between a positive electrode active material and a solid electrolyte material. For example, Narumi Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007), pages 1486 to 1490 describes a material in which the surface of LiCoO2 (positive electrode active material) is coated with LiNbO3 (lithium niobate). This technique attempts to obtain a high-power battery in such a manner that the surface of LiCoO2 is coated with LiNbO3 to suppress reaction between LiCoO2 and the solid electrolyte material to thereby reduce the interface resistance between LiCoO2 and the solid electrolyte material.
In addition, it is also known that a negative electrode active material reacts with a solid electrolyte material to form a high-resistance portion on the surface of the negative electrode active material and, therefore, interface resistance against movement of lithium ions across the interface between the negative electrode active material and the solid electrolyte material increases (see Japanese Patent Application Publication No. 2004-206942 (JP-A-2004-206942)). In order to solve the above problem, JP-A-2004-206942 describes an all-solid battery in which a second solid electrolyte layer that has an ion conductivity lower than that of a first solid electrolyte layer (sulfide-based solid electrolyte material) and that does not chemically react with the first solid electrolyte is formed between the first solid electrolyte layer and a negative electrode made of metal lithium. This technique is to form the second solid electrolyte layer to thereby suppress reaction between the sulfide-based solid electrolyte material and the metal lithium.
Narumi Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007), pages 1486 to 1490 describes a method of coating the surface of LiCoO2 with LiNbO3 in such a manner that a coating solution that contains the precursor of LiNbO3 is applied onto the surface of LiCoO2 (positive electrode active material) and is then subjected to heat treatment in an oxygen atmosphere. In order to suppress reaction between the positive electrode active material and the solid electrolyte material, it is necessary to coat the surfaces of the positive electrode active material particles with LiNbO3, and, in addition, it is desirable to uniformly coat the surfaces of the positive electrode active material particles with thin LiNbO3. The above method may be rolling fluidized coating. However, the rolling fluidized coating is a batch treatment, and the apparatus is large, so there is a problem that manufacturing cost is high. In addition, LiNbO3 has a weak bonding force to the surface of LiCoO2 (positive electrode active material) and easily peels off when it receives shearing force. Because the rolling fluidized coating apparatus has an aggregate crushing mechanism (screen), so it is difficult to completely suppress peeling of LiNbO3.